Internal combustion engine supplying gaseous fuel and control method thereof

Abstract

An internal combustion engine supplying gaseous fuel, in particular hydrogen, comprises, for each cylinder (1), a first intake duct (3A) and a second intake duct (3B) controlled by a first intake valve (VA) and a second intake valve (VB), respectively. In each operating cycle of the cylinder, first only the opening and closing movement of the first intake valve (VA) is activated while the second intake valve (VB) remains closed, and successively only the opening and closing movement of the second intake valve (VB) is activated while the first intake valve (VA) remains closed. According to the present invention, due to the cooling obtained during the opening of the first intake valve, gaseous fuel is directly supplied into the cylinder only from the moment after the first intake valve (VA) is closed and before the second intake valve (VB) is opened, thereby enabling regular combustion without a pre-ignition phenomenon or a tempering phenomenon.

Description

Technical Field

[0001] The present invention relates to an internal combustion engine supplied with gaseous fuel.

[0002] In particular, the invention relates to an internal combustion engine supplied with gaseous fuel, of the type comprising one or more cylinders and respective pistons sliding in the cylinders between top dead center (TDC) and bottom dead center (BDC) and operatively connected to a crankshaft, the engine being configured to actuate successive intake, compression, expansion and exhaust phases during each operating cycle of each cylinder,

[0003] Wherein, for each cylinder, the engine includes:

[0004] - a first intake duct and a second intake duct, which open into the cylinders and both communicate with the same intake manifold so as to receive air at the same pressure;

[0005] a first intake valve and a second intake valve associated with the cylinder to control the flow of intake air into the cylinder from the first intake duct and from the second intake duct, respectively, during each operating cycle of the cylinder;

[0006] - an actuation device for actuating said first intake valve and said second intake valve so as to control an opening movement and a subsequent closing movement of said first intake valve and said second intake valve during each operating cycle of the cylinder;

[0007] - A gaseous fuel supply system comprising at least one injector associated with each cylinder and configured and arranged to inject gaseous fuel directly into the respective cylinder during each operating cycle of the cylinder. Background Art

[0008] In engines of the above-mentioned type, there is the problem of preventing backfire phenomena in the intake duct and knocking and / or pre-ignition phenomena in the combustion chamber.

[0009] A known method for solving the above problem is to use non-combustible gas (fresh air) to cool the exhaust gas remaining in the cylinder at the beginning of the intake phase. In this way, the fuel mixture is prevented from being in a high temperature environment.

[0010] Another way to eliminate backfire is to inject the gaseous fuel directly (DI) into the combustion chamber instead of injecting it into the intake duct, the so-called port fuel injection (PFI). However, in the case of ultra-lean combustion, which is useful for increasing thermal efficiency, in the case of PFI as in the case of DI, misfires may occur, resulting in backfire.

[0011] In the case where the gaseous fuel is hydrogen, it should also be taken into account that the minimum ignition energy of hydrogen is approximately ten times smaller than that of gasoline. This characteristic, combined with the tendency of hydrogen to burn under sufficient restrictions of flammability and high diffusivity, leads to a sharp increase in the risk of flashback in the intake duct. Countermeasures that are usually effective for gasoline-fired engines or natural gas-fired engines may therefore be ineffective in the case of engines supplied with hydrogen.

[0012] Furthermore, hydrogen-fueled engines need to be operated with an ultra-lean mixture to prevent nitrogen oxide emissions and knock phenomena (due to the low ignition energy of hydrogen). Lean mixtures usually burn more slowly, which means that when the intake valve opens, there may still be a certain amount of air and unburned fuel in the cylinder, which can lead to backfire. In addition, for engines operating in the presence of an ultra-lean mixture, usually the compression ratio is increased to promote ignition and complete combustion: however, during the start of combustion, the high compression ratio means that the turbulent kinetic energy at top dead center is significantly reduced, increasing the risk of slow and incomplete combustion.

[0013] An attempt to solve this problem has been proposed in document US 7,980,220 B2. In this known solution, an engine of the above-mentioned type is provided, each cylinder having two intake ducts and two corresponding intake valves. The injection of hydrogen takes place in only one of the two intake ducts. At least at higher engine loads and rpm, during each operating cycle of the cylinder, the intake valve of the intake duct into which the hydrogen is injected is opened with a delay relative to the other intake valve. In this way, only fresh air enters the combustion chamber during the steps in which only the intake valve of the intake duct not associated with the injector is opened. In theory, the above-mentioned solution should eliminate the risk of flashback due to the cooling performed by the fresh air introduced in the first part of the intake phase. However, a solution of the latter type is not entirely satisfactory, since the above-mentioned cooling takes place only during the first part of the intake phase. In addition, the vortex initially generated may cause the flow of air and hydrogen introduced by opening the second valve to precisely impinge on the hot exhaust valve, with the risk of flashback, i.e. undesired ignition of the fuel during the intake phase.

[0014] Document US 2012 / 023935 A1 describes a conventional internal combustion engine supplied with liquid fuel, in which two intake ducts per cylinder supply air flows with different temperatures and pressures.

[0015] Purpose of the Invention

[0016] The object of the present invention is to provide an internal combustion engine supplied with gaseous fuel in which the risk of flashback is significantly reduced.

[0017] A further object is to provide an internal combustion engine supplied with a gaseous fuel, in particular hydrogen, in which the risk of knocking and pre-ignition (so-called macro-knock phenomena) is also significantly reduced.

[0018] In particular, the object of the present invention is to provide an internal combustion engine supplied with gaseous fuel which will enable effective cooling of hot spots in the combustion chamber to prevent undesired ignition of the fuel during the opening of the intake valve or valves, i.e. to prevent flashback or irregular ignition phenomena.

[0019] Furthermore, another object of the present invention is to provide an internal combustion engine supplied with gaseous fuel in which nitrogen oxide emissions will be significantly reduced, also providing the possibility of operating with an ultra-lean mixture (i.e. the ratio between the air / fuel ratio and the stoichiometric ratio is higher than a threshold value, such as 1.8), taking advantage of the increase in turbulent kinetic energy in the cylinder, thus causing better homogeneity of the air / fuel mixture. Summary of the invention

[0020] In order to achieve one or more of the above-mentioned objects, the subject of the invention is an internal combustion engine supplied with gaseous fuel, of the type indicated at the beginning of this description, and characterized in that:

[0021] the actuating means for actuating the first intake valve and the second intake valve being configured to, in each operating cycle of the cylinder, at least under given engine operating conditions, firstly control only the opening movement and the successive closing movement of the first intake valve, while the second intake valve remains closed, and then control only the opening movement and the successive closing movement of the second intake valve, while the first intake valve remains closed; and

[0022] The gas fuel supply system is configured such that:

[0023] In each operation cycle of each cylinder, when the first intake valve is opened, gaseous fuel is not supplied, so that the air flow introduced into the cylinder with the first intake valve opened causes cooling of the cylinder, thereby preventing subsequent pre-ignition or flashback phenomena, while

[0024] In each operating cycle of the cylinder, due to the cooling obtained during the opening of the first intake valve, the gaseous fuel is directly supplied to the cylinder only from the moment after the first intake valve is closed and before the second intake valve is opened, thereby obtaining regular combustion without pre-ignition or backfire.

[0025] In particular, for example, if the fuel injector is in a coaxial position relative to the cylinder, the fuel will remain confined near the nozzle of the injector due to the strong swirl motion, thus reducing the risk of inducing irregular ignition, since the fuel will not come into contact with the hot spot of the combustion chamber located in the area of ​​the exhaust valve. The sequential and delayed opening of the second intake valve allows the fuel charge to mix with the air in the cylinder, thus ensuring the homogeneity necessary to prevent the formation of nitrogen oxides.

[0026] Due to the above characteristics, the cylinder receives only fresh air during the entire phase of the opening of the first intake valve. The swirling motion of the air flow introduced into the cylinder during the opening of the first intake valve has a significant positive effect on the cooling of the hot spots of the combustion chamber, thereby reducing the tendency to knock. The reduction in the risk of knock enables operation at a higher compression ratio, which provides benefits in terms of thermal efficiency.

[0027] Fuel injection, in particular using the centrally located injector, occurs after the first valve has closed and prevents the fuel from reaching hot spots in the combustion chamber, thereby preventing any undesired ignition and flashback.

[0028] The consecutive opening steps of the first and second intake valves cause a combination of swirl and tumble motions of the air flow, which, besides ensuring perfect mixing of the injected fuel with the air, reduces the risk of a backflow of gaseous fuel into the supply duct and accordingly reduces the risk of flashback.

[0029] In particular, at the end of the intake phase, since the two intake valves open in separate and consecutive steps, the movement of the swirl, which can be adjusted according to the engine operating point, has an important influence on the homogeneity of the air / fuel mixture, which in turn reduces nitrogen oxide emissions, and reduces heat losses during combustion (on the contrary, too pronounced swirl would increase heat losses due to the contact of the flow with the walls of the combustion chamber).

[0030] Research and experiments conducted by the applicant have shown that opening the two intake valves per cylinder of an engine at subsequent times, interspersed with fuel injection, can generally:

[0031] Cooling hot spots in the combustion chamber to prevent flashback and unwanted ignition; and

[0032] The intensity of the macroscopic movements carried out inside the combustion chamber, namely the so-called swirl movement (flow rotating around the axis of the cylinder), tumble movement (flow rotating around an axis perpendicular to the axis of the cylinder) and cross-tumble movement (flow rotating around an axis perpendicular to the axis of the cylinder and perpendicular to the axis of the tumble), is adjusted in order to favor the uniformity of the charge. The above advantages derive from the fact that the two intake ducts flow into the cylinder in a tangential direction relative to the cylinder axis. In the initial phase of the intake stroke, starting when the piston is approximately at TDC, only the first intake valve is open. The air flow introduced into the cylinder through the first intake duct generates a fluid dynamic field including a swirl component in addition to the tumble and cross-tumble components. When the second intake valve opens after the first intake valve is closed, a dual effect occurs:

[0033] 1) Generates a motion field that is opposite to the component of the motion field of the vortex generated by the opening of the first intake valve, so that the pre-existing vortex weakens or even reverses its direction of rotation. In addition, the vortex generated by the opening of the first valve is also transformed into tumble and cross tumble, which facilitates the mixing of the injected fuel with the air in order to have a homogeneous mixture during combustion and prevents the formation of nitrogen oxides or any irregular combustion. This phenomenon is particularly desirable at higher engine loads, because the risk of irregular ignition is higher (higher temperatures of the walls of the combustion chamber) and because it is more difficult to ensure the homogeneity of the mixture due to the larger amount of fuel.

[0034] 2) Furthermore, only opening the second intake valve after closing the first intake valve generates a new amount of turbulence which, although this turbulence dissipates, leads to a higher total content of turbulent kinetic energy at TDC than in the case of simultaneous conventional actuation of the intake valves. The dissipation of turbulence is proportional to the cube of the intensity of the turbulence itself: the generation of a large amount of turbulence at the beginning of the intake phase leads to a significant dissipation of the turbulence. The delayed generation of new turbulence makes it possible to increase the amount of turbulence at the ignition angle. Ideally, the available turbulent kinetic energy at the ignition angle could be doubled compared to the case of conventional actuation of the intake valves, which has obvious benefits for greater stability of the combustion and therefore for increasing the possibility of, for example, charge dilution. In particular, in the case of ultra-lean combustion, greater turbulence is required.

[0035] In an example, during each operating cycle of the cylinder, the gaseous fuel supply system is configured to complete the supply of gaseous fuel before the second intake valve opens. In another example, during each operating cycle of the cylinder, the gaseous fuel supply system is configured to complete the supply of gaseous fuel after the second intake valve has opened. During each operating cycle of the cylinder, the supply of gaseous fuel may be performed via a single injection or via multiple injections.

[0036] In one embodiment, the engine includes an actuating device for variable actuation of a first intake valve and a second intake valve, the actuating device being configured to obtain a first operating mode or a second operating mode or a third operating mode under different engine operating conditions, in which first operating mode, in a continuous period of time, first only the first intake valve is opened, and then only the second intake valve is opened, in which second operating mode, the first intake valve and the second intake valve are opened at the same or relatively close crank angles to each other, and the first intake valve and the second intake valve are closed at the same or relatively close crank angles to each other, and in which third operating mode, only the second intake valve is opened and closed.

[0037] The device for variable actuation of the intake valve may be an electronically controlled hydraulic actuation device, or a device having an electromagnetic or electropneumatic actuator, or a device comprising an actuation cam having different profiles which can be selectively actuated.

[0038] Yet another object of the present invention is the above engine control method.

[0039] Further advantageous features of the invention are pointed out in the dependent claims. DETAILED DESCRIPTION

[0040] Further features and advantages of the invention will emerge from the ensuing description, with reference to the accompanying drawings, provided purely as non-limiting examples, in which:

[0041] Figure 1 and Figure 2 is a perspective view and a top plan view of a combustion chamber of a cylinder of an internal combustion engine according to the present invention;

[0042] Figure 3 is a perspective view of a conventional type of system for actuating an intake valve associated with a cylinder of a conventional internal combustion engine;

[0043] Figure 3A Shows Figure 3 corresponding to a modification of the example of embodiment of the engine according to the invention;

[0044] Figure 4-Figure 6 is a schematic diagram showing various examples of opening and closing cycles of two intake valves associated with a cylinder of an engine according to the present invention, compared with a conventional cycle;

[0045] Figure 7-Figure 11 is a schematic diagram illustrating the advantages of the present invention; and

[0046] Fig.12 is a schematic diagram of a system for variable actuation of an intake valve of a known type developed by the applicant and sold under the trademark MultiAir, which may be used in embodiments of the present invention.

[0047] Reference Figure 1 and Figure 2 , reference numeral 2 denotes as a whole a combustion chamber associated with a cylinder 1 of an internal combustion engine supplied with hydrogen according to the invention.

[0048] Figure 1 and Figure 2 Two intake ducts 3A, 3B are shown, which discharge into the combustion chamber 2 and are shaped and arranged according to any known technique so as to introduce a respective air flow into the cylinder in a direction set at a distance from the axis C1 of the cylinder. Both ducts 3A, 3B communicate with the same intake manifold 30 (only partially shown) and respectively receive air at the same pressure from the line for supplying air to the engine. Figure 1 and Figure 2 Also shown are two exhaust ducts 4A, 4B which are associated with the cylinder 1 and which converge in an exhaust manifold 5 of the internal combustion engine (at Figure 2 The middle part is visible).

[0049] The engine according to the invention is provided with a hydrogen supply system of any known type. The hydrogen supply system comprises a hydrogen injector H2 for each cylinder, such as Figure 2 As shown, it is arranged and configured to supply hydrogen directly into the cylinder. Injector H2 is only Figure 2 Schematically shown in the drawings, as it can be manufactured according to any known technology. Only the parts relevant to the purpose of the invention are shown in the drawings, it being understood that the structure and the general construction of the engine can also be obtained in any known manner. In particular, the drawings do not show the spark plug associated with the combustion chamber 2.

[0050] Associated with the two intake ducts 3A, 3B are two conventional mushroom-type intake valves having valve stems and rounded heads: a first intake valve VA and a second intake valve VB.

[0051] As will be explained further below, for the purposes of the present invention, the system for actuating the intake valves VA, VB may be of any known type. Figure 3 A device for actuating intake valves VA, VB of conventional type is shown, comprising a camshaft 6 which is rotatably supported in the structure of the cylinder head of the engine and which is controlled in rotation in a conventional manner by a crankshaft (not shown) of the internal combustion engine by means of a transmission, for example a toothed belt transmission. Figure 3 Only a part of the camshaft is shown in FIG. 1 ) includes two cams 6A and 6B, which are used to actuate the first intake valve VA and the second intake valve VB respectively. Figure 3In the conventional example shown, two cams 6A, 6B actuate two valves VA, VB by means of respective rockers 7A, 7B, each of which is mounted in a swinging manner at one end on a support 8 carried by the structure of the cylinder head and the opposite end acts on a respective intake valve.

[0052] In the case of the first embodiment of the present invention, the cams 6A, 6B are Figure 3A Modify the method shown to obtain Figure 4-Figure 6 A valve lift curve of the type shown in any of the accompanying drawings.

[0053] Figure 4 Valve lift curves of two intake valves VA, VB according to a first example of an embodiment of the present invention are shown. Figure 4 The diagram shows the displacement of each intake valve as a function of the engine rotation angle. According to the convention adopted here, an engine rotation angle equal to 360° corresponds to the state of the piston in the cylinder when it is at TDC. The position of the piston at BDC corresponds to a crank angle of 540°.

[0054] exist Figure 4 , the line LC represents the valve lift diagram of the intake valves in the case of a conventional engine provided with a conventional cam. In the case of the conventional solution, the two intake valves VA, VB are controlled simultaneously and synchronously according to the profile LC. It can be seen that in the conventional solution, the two valves start to open immediately before TDC, reach a maximum open state near a crank angle of approximately 470°, and close again at a crank angle of approximately 600°. In order to achieve this result, the two cams 6A, 6B have the same profile, such as to produce the valve lift curve LC, and have the same angular position on the camshaft 6, such as Figure 3 shown.

[0055] exist Figure 4 In the example of the embodiment of the present invention shown, the cams 6A, 6B have different configurations from each other (eg Figure 3A ) and are oriented at different angles on the camshaft 6. The two cams 6A, 6B are constructed and oriented so that the inlet valves VA, VB are respectively generated by Figure 4 LA and LB in the figure represent the valve lift curve (it should be noted that Figure 4 The valve lift curves LA and LB in the following figures are merely qualitative).

[0056] The first important feature to note is that during each operating cycle of the cylinder, first only the opening and closing movement of the first intake valve VA is activated, while the second intake valve VB remains closed, and then only the opening and closing movement of the second intake valve VB is activated, while the first intake valve VA remains closed.

[0057] The second important feature to be noted is that the hydrogen supply system is controlled so that in each operating cycle of each cylinder, no hydrogen is supplied when the first intake valve VA is open, so that the air flow introduced into the cylinder with the first intake valve VA open is used to cool the cylinder by utilizing the swirl motion of the air flow, thereby preventing successive pre-ignition or flashback phenomena, whereas, in each operating cycle of the cylinder, hydrogen is directly supplied to the cylinder through the injector H2 only from the moment after the first intake valve is closed and before the second intake valve is opened. Figure 4 , the step of activating the hydrogen injector H2 is indicated by the dashed line IH2. In the example shown, it is conceivable that the supply of hydrogen will start after the closing of the intake valve VA and will stop before the opening of the intake valve VB. However, it is conceivable that the supply of hydrogen, which can be performed with a single injection or by multiple injections, will always start after the closing of the intake valve VA, but will stop after the opening of the second intake valve VB.

[0058] Due to the cooling obtained during the opening of the first intake valve, the fresh air flow entering the cylinder only during the opening of the first intake valve VA produces effective cooling of the combustion chamber, which causes regular combustion without pre-ignition phenomena or flashback phenomena.

[0059] Figure 4 A particularly preferred example relates to an embodiment in which the first intake valve VA begins to open when the piston in the cylinder approaches TDC (or close to TDC, immediately before TDC or immediately after TDC), and then closes before a crank angle of 540° (i.e., when the piston in the cylinder is still moving in the BDC direction and has not yet reached BDC). In contrast, the second intake valve VB opens after the piston in the cylinder has reached BDC and has returned toward TDC (e.g., a crank angle of approximately 560°), and closes after a further rotation of the crankshaft (e.g., approximately 90°).

[0060] Refer again Figure 4 It can be noted that, according to the conventional valve lift curve LC, the maximum valve lift of the first intake valve VA is approximately 80% of the maximum valve lift of the conventional cycle in which the intake valves VA and VB are opened and closed simultaneously, while the maximum valve lift of the second intake valve VB is approximately 3 / 8 of the maximum valve lift of the first intake valve VA (or according to the profile LC, approximately 30% of the maximum conventional valve lift).

[0061] Figure 5 and Figure 6 is similar to Figure 4 Schematic diagram showing other examples of embodiments of the present invention, which differ from Figure 4 .

[0062] All the aforementioned embodiments have in common that during the intake phase in the cylinder, in a first period substantially only the first intake valve VA opens while the second intake valve VB remains closed, and in a second period only the second intake valve VB opens while the first intake valve VA remains closed.

[0063] The aforementioned features enable to obtain the advantage of increased turbulent kinetic energy (TKE) in the air charge introduced into the cylinder, regardless of the phase of the opening and closing cycles of the two intake valves. This result is related to the arrangement of the intake duct in a direction set at a certain distance from the axis of the cylinder. Therefore, when only the air from the intake duct 3A is introduced into the cylinder, the introduced air flow generates a fluid dynamic field with a vortex component (a flow rotating around the axis C1 of the cylinder). Then, when the first intake valve VA is closed and the second intake valve VB is opened, the flow from the other intake duct 3B disturbs the previously generated fluid dynamic field, thereby increasing its TKE. Once the mixture of air and fuel explodes (after ignition with a spark plug in the case of a gasoline-fired engine, or after compression in the cylinder in the case of a diesel engine), the increase in turbulent kinetic energy in the cylinder leads to an increase in the propagation speed of combustion.

[0064] The above advantages of increasing TKE in the air flow introduced into the cylinder are Figure 4 , Figure 5 and Figure 6 In the case of the embodiments of the present invention, the pressure of the second intake duct 3B is multiplied. In this case, in fact, when the first intake valve VA is closed, the piston in the cylinder continues to descend in the direction of BDC, establishing a negative pressure in the cylinder. In the case of these embodiments, the second intake valve is opened after BDC (i.e. when the piston has returned towards TDC). Nevertheless, the negative pressure field previously established in the cylinder means that the step in which the second intake valve VB is opened will also cause air to actively enter the cylinder from the second intake duct 3B, while generating a high TKE in the fluid dynamic field inside the cylinder.

[0065] As already mentioned above, the invention can be implemented both with an internal combustion engine having a device for actuating intake valves of a conventional type in which the valve lift curves of the two intake valves are predetermined and fixed, and with an internal combustion engine equipped with a system for variable actuation of the intake valves.

[0066] So, for example, reference Figure 6The cam 6A causing the opening and closing of the first intake valve VA can be controlled by means (for example electrohydraulic means) so as to have a profile to determine the valve lift curve indicated by LA1. In fact, due to the separation of the movement of the cam from the movement of the intake valve by hydraulic means, the movement of the intake valve may have a non-monotonic regularity. However, the variable actuation system itself can be used to vary the lift curve of the valve VA, for example according to the profile LA2. Similarly, the actuation means combined with the cam 6B can be used according to Figure 6 The valve lift curve LB1 shown produces a regularity of motion. However, the engine may be equipped with a variable actuation system capable of obtaining an effective lift of the valve VB according to the profile LB2.

[0067] In an example, the invention is applied to an internal combustion engine equipped with a system for variable actuation of the intake valves of the engine of the type developed by the Applicant and sold under the trademark MultiAir.

[0068] Fig.12 is a schematic diagram of an example of a MultiAir variable actuation system. In this case, each intake valve VA, VB ( Fig.12 The device associated with the valve VA is shown to be actuated by a corresponding cam 6A or cam 6B by means of an electronically controlled hydraulic device 8. The cam 6 actuates a tappet 9 which is kept in contact with the cam 6 by a return spring 10. The tappet 9 is associated with a pumping plunger 11 of a master cylinder which delivers fluid at the pressure of a chamber 12 to a chamber of a slave cylinder 13, the plunger 14 of which serves as actuator for the intake valve VA. The intake valve VA is pulled by a spring 15 towards a closed position of the intake duct 3A. All the aforementioned components are carried by a structure 16 of the cylinder head of the engine. The solenoid valve 17 is controlled by an electronic control unit E. When the solenoid valve is in the closed state, it interrupts the communication between the pressurized fluid chamber 12 and a low-pressure environment 18, which is in communication with a fluid accumulator 19 and with an inlet 20 intended to be connected to the lubrication circuit of the engine. If the solenoid valve 17 is in the closed state, the pressurized fluid chamber 12 is isolated so that the movement applied to the tappet 9 by the cam 6 can be transmitted to the intake valve VA via the fluid in the chamber 12 and the slave cylinder 13. In the case where the cam 6 keeps the intake valve VA open, the opening of the solenoid valve 17 controlled by the electronic control unit E causes the discharge of the pressurized fluid chamber 12 and the intake valve VA is subsequently closed due to the action of the return spring 15. In this case, the intake valve VA is insensitive to the movement applied to the tappet 9 by the cam 6.

[0069] This description is provided here only to illustrate the basic working principle of the MultiAir system.The applicant has developed various embodiments of the aforementioned system, which form the subject of various patent publications, including those already mentioned above.

[0070] It will be appreciated that the invention may also be used in conjunction with any known type of variable actuation system for an intake valve, such as an electromagnetic actuation system, or other variable actuation systems of the type comprising, for example, a multi-profile cam.

[0071] In the case of a variable actuation system, it is possible that the above-described operating mode, i.e., first only the first intake valve and then only the second intake valve is actuated for a continuous time, is implemented only under given engine operating conditions, while under other engine operating conditions, the two intake valves per cylinder are controlled in a conventional manner so that they open and close simultaneously.

[0072] Figures 7 to 11 It shows the main benefits that can be obtained with the present invention compared to a conventional standard embodiment where both intake valves are open at TDC and closed at BDC.

[0073] Figure 7 : is a graph showing the change in the average value of turbulent kinetic energy (TKE) in the combustion chamber in the case of the standard example and in the case of the present invention. Figure 7 The diagram shows how the delayed opening of the second intake valve creates a new increase in turbulent kinetic energy near BDC, so that even though TKE dissipates, at the subsequent TDC the value of TKE will be much higher than in the standard case. This higher TKE both ensures perfect mixing of fuel and air and supports combustion in the case of ultra-lean mixtures.

[0074] Figure 8 a. Figure 8 b and Figure 8 c is a schematic diagram of the various organized macroscopic motions occurring in the combustion chamber. The above figure shows a reference system in which the axis X is located in the plane of symmetry of the duct of the intake valve and coincides with the air introduced into the combustion chamber. Therefore, what is defined as "tumble motion" is the motion located in a plane orthogonal to the unit vector Y; what is defined as "cross tumble motion" is the motion located in a plane orthogonal to the unit vector X; what is defined as "vortex motion" is the motion located in a plane orthogonal to the unit vector Z. The so-called tumble, cross tumble and vortex are defined as follows:

[0075] Tumble index = ω 滚流 / ω 发动机

[0076] Cross tumble index = ω 交叉滚流 / ω 发动机

[0077] Eddy current index = ω 涡流 / ω 发动机

[0078] Among them, ω 发动机is the rotational speed of the engine, ω 滚流 ,ω 交叉滚流 and ω 涡流 is the average angular velocity of the corresponding motion in radians per second.

[0079] Fig. 9 The effect of the opening of the second intake valve on the swirl generated by the previous opening cycle of the first intake valve is shown: the dashed line refers to the diagram that the swirl index would have presented after the opening movement of the first intake valve, without the second intake valve opening. It can be noted that the intensity of the swirl during combustion is also considerable, as well as the increased heat exchange and the deterioration of the engine efficiency. A further and most important disadvantage of the generally swirl-type motion field for engines with direct fuel injection, especially with a centrally located injector, is that a uniform mixing of the air / fuel mixture cannot be guaranteed. The solid line shows the advantage of the invention with respect to the swirl: as soon as the second intake valve starts to open, the swirl decreases in a manner proportional to the air introduced during the opening of the second intake valve. It can be inferred that having an effective device for actuating the intake valves capable of varying the curve of the opening movement of the second intake valve makes it possible to adjust the intensity of the swirl present in the chamber during combustion. In this way, two effects are obtained:

[0080] 1. The homogeneity of the air / fuel charge is improved due to the reversal and / or decay of the swirl; and 2. The reduced swirl reduces heat exchange during the combustion phase, improving efficiency.

[0081] The dash-dot line refers to a standard embodiment in which the formation of eddies is obviously not envisaged.

[0082] Fig.10 The effect of the opening of the second intake valve on the tumble flow generated by the previous opening cycle of the first intake valve is shown: It should be noted that at the beginning of the opening of the second intake valve (which occurs at 500° crank angle in this example), the swirl index suddenly decreases (at Fig. 9 In a manner similar to the standard case, starting from 660° crank angle, the tumble index is eliminated due to the movement of the piston compressing the tumble flow, with the associated conversion of kinetic energy into turbulent kinetic energy: Figure 7 As shown, from a crank angle of 660° to a crank angle of 700°, TKE is no longer dissipated further, because the dissipated TKE is replaced by the dissipation caused by the elimination of tumble flow. The transition from swirl to tumble flow after fuel injection occurs after the first valve is closed, which is beneficial to the homogeneity of the air / fuel charge, which has great benefits for nitrogen oxide emissions and irregular combustion.

[0083] Fig.11 A graph showing the cross tumble index: Similar to vortex flow ( Fig. 9), which does not exist in the case of standard actuation. And similarly to the case of tumble motion ( Fig.10 ), due to the conversion of the swirl motion, after the opening motion of the first intake valve, the cross tumble motion is accelerated when the second intake valve is opened. Similar to the case of tumble motion, the cross tumble motion has two main advantages:

[0084] 1. The cross-tumble motion helps maintain TKE over the crank angle range between 660° and 700°; and

[0085] 2. In combination with the tumble motion, the high cross tumble motion (which is absent in the case of standard actuation) facilitates a homogeneous mixing of the injected fuel with the air present in the cylinder.

[0086] In summary, it is possible to understand how the modularity of the vortex affects the other organized movements: thus, by varying the parameters that define the two consecutive actuations of the first and second intake valves, it is possible to optimize the requirements for initial cooling of the combustion chamber, for optimal air / fuel mixing, and for obtaining high TKE values ​​at TDC.

[0087] In an embodiment, the dimensions of the first intake duct and the second intake duct are set so that under high engine load, closing of the first intake valve generates a pressure wave that climbs up along the first intake duct and enters the second intake duct from the common intake manifold to maximize the charge to the cylinder.

[0088] In another example, the first intake duct and the second intake duct have different diameters and different lengths, which are selected in such a way that, in the case of maximum filling of the engine and complete opening of the second intake valve, the air flow entering the combustion chamber with the opening of the second intake valve does not offset the swirl motion of the air flow previously introduced into the combustion chamber with the opening of the first intake valve.

[0089] In another example, the device for actuating the first intake valve and the second intake valve is configured to control the valve lift of the first intake valve to be significantly smaller than the valve lift of the second intake valve, so that the filling of the cylinder is mainly obtained due to the opening of the second intake valve.

[0090] In the engine according to the invention, the above advantages are added to those related to reduced risk of flashback, reduced nitrogen oxide emissions and reduced risk of pre-ignition and knocking which would otherwise occur as a result of hydrogenated fuel.

[0091] It should be noted that, as described above, when the piston has not yet reached BDC, in particular when the piston is approximately halfway through its intake stroke, the first intake valve is closed so that the subsequent movement of the piston generates a negative pressure in the cylinder that facilitates the entry of the flow from the second intake duct when the second intake valve opens. The longer the delay of the opening of the second intake valve VB relative to the closing of the first intake valve VA, and the greater the negative pressure generated in the cylinder with the movement of the piston after the closing of the first intake valve VA, the greater the flow generated by the opening of the second intake valve VB. A greater flow corresponds to a higher momentum and to a higher turbulent kinetic energy, which facilitates proper air / fuel mixing.

[0092] In the case of a device for variable actuation of the intake valve, the degree to which the second intake valve VB is opened can be adjusted in order to adjust the intensity of the swirl motion in the combustion chamber after the second intake valve is opened. The adjustment of the swirl motion has a significant effect on the other components of the flow motion in the combustion chamber (tumble and cross tumble) and on the instantaneous generation of turbulent kinetic energy, which in turn causes an increase in the combustion rate.

[0093] Therefore, the optimal value of the swirl intensity which can be adjusted by adjusting the valve lift curve of the second intake valve depends on various factors, in particular: on the gaseous fuel injection system (PFI or DI); and in the case of direct injection (DI), on the position of the injector in the combustion chamber (lateral or central); on the crank angle at the start of gaseous fuel injection and at the end of gaseous fuel injection; and on the engine load and the desired type of combustion, whether stratified or homogeneous charge.

[0094] The significant swirl generated in the cylinder when the first intake valve is open has a drastic cooling effect on hot spots in the combustion chamber.

[0095] In the case of use of a device for variable actuation of the intake valves, the advantage is achieved that the valve lift curve for each valve can be quasi-rectangular with a particularly high valve closing speed, which is particularly important for achieving the advantages of the invention.

[0096] Of course, without prejudice to the principle of the invention, the embodiments and the construction details may vary widely with respect to what has been described and illustrated purely by way of example, without thereby departing from the scope of the invention as defined in the appended claims.

Claims

1. An internal combustion engine supplied with gaseous fuel, in particular hydrogen, comprising one or more cylinders (1) and respective pistons sliding in said cylinders between TDC and BDC and operatively connected to a crankshaft, said internal combustion engine being configured to actuate successive phases of intake, compression, expansion and exhaust during each operating cycle of each cylinder, in, The internal combustion engine comprises, for each cylinder: A first intake conduit (3A) and a second intake conduit (3B), the first intake conduit and the second intake conduit leading to the cylinder and both communicating with the same intake manifold (30) to receive air at the same pressure; a first intake valve (VA) and a second intake valve (VB) associated with the cylinder (1) to control the flow of intake air from the first intake duct (3A) and from the second intake duct (3B), respectively, into the cylinder during each operating cycle of the cylinder; an actuating device (6) for actuating the first intake valve (VA) and the second intake valve (VB) to control the opening and subsequent closing movements of the first intake valve (VA) and the second intake valve (VB) during each operating cycle of the cylinder; and a gaseous fuel supply system comprising at least one injector (H2) associated with each cylinder and configured and arranged to inject gaseous fuel directly into the respective cylinder during each operating cycle of the cylinder, The internal combustion engine is characterized in that: The actuating device for actuating the first intake valve (VA) and the second intake valve (VB) is configured to, in each operating cycle of the cylinder, at least under some engine operating conditions, firstly control only the opening movement and the successive closing movement of the first intake valve (VA), while the second intake valve (VB) remains closed, and then control only the opening movement and the successive closing movement of the second intake valve (VB), while the first intake valve (VA) remains closed; and The gas fuel supply system is configured such that: In each operating cycle of each cylinder (1), when the first intake valve (VA) is open, no gaseous fuel is supplied, so that the air flow introduced into the cylinder with the first intake valve (VA) open causes cooling of the cylinder, thereby preventing subsequent pre-ignition or back-ignition phenomena, and In each operating cycle of the cylinder (1), due to the cooling obtained during the opening of the first intake valve (VA), gaseous fuel is directly supplied to the cylinder from the moment after the first intake valve (VA) is closed, without any pre-ignition or backfire.

2. The internal combustion engine according to claim 1, characterized in that The gaseous fuel supply system is configured to complete supply of gaseous fuel before the second intake valve (VB) opens during each operating cycle of the cylinder.

3. The internal combustion engine according to claim 1, characterized in that The gaseous fuel supply system is configured to complete the supply of gaseous fuel after the second intake valve (VB) has opened during each operating cycle of the cylinder.

4. The internal combustion engine according to claim 1, characterized in that The internal combustion engine includes a variable actuation device (8) for the first intake valve (VA) and the second intake valve (VB), and wherein the variable actuation device (8) is configured to provide a first operating mode, a second operating mode, or a third operating mode under different engine operating conditions, in which in the first operating mode, in a continuous period of time, first only the first intake valve (VA) is opened, and then only the second intake valve (VB) is opened, in the second operating mode, the first intake valve (VA) and the second intake valve (VB) are opened at the same crank angle or crank angles relatively close to each other, and the first intake valve (VA) and the second intake valve (VB) are closed at the same crank angle or crank angles relatively close to each other, and in the third operating mode, only the second intake valve (VB) is opened and closed.

5. The internal combustion engine according to claim 3, characterized in that The aforementioned first operating mode is implemented when the internal combustion engine is operated at a load above a preset threshold.

6. The internal combustion engine according to claim 1, wherein: The midpoint of the opening phase of the first intake valve (VA) is located in the first half of the intake stroke of the piston from TDC to BDC, while the midpoint of the opening phase of the second intake valve (VB) is located in the compression stroke of the piston from BDC to TDC.

7. The internal combustion engine according to claim 4, wherein: The variable actuation device of the first intake valve (VA) and the second intake valve (VB) is an electronically controlled hydraulic actuation device (8), or a device having an electromagnetic actuator or an electropneumatic actuator, or a device including an actuation cam having different profiles that can be selectively actuated.

8. The internal combustion engine according to claim 1, characterized in that The gaseous fuel supply system is configured to complete supply of gaseous fuel before the second intake valve (VB) is closed during each operating cycle of the cylinder.

9. The internal combustion engine according to claim 1, characterized in that The gas fuel injector (H2) is set in a coaxial position relative to the cylinder (1).

10. The internal combustion engine according to claim 3, characterized in that The aforementioned first operating mode is implemented when the internal combustion engine is operated with an ultra-lean mixture, ie, with an ultra-lean mixture in which the ratio between the air / fuel ratio and the stoichiometric ratio is above a threshold value, for example 1.

8.

11. A method for controlling the operation of an internal combustion engine comprising one or more cylinders (1) and respective pistons sliding in said cylinders between TDC and BDC and operatively connected to a crankshaft, wherein: The successive phases of intake, compression, expansion and exhaust are provided during each operating cycle of each cylinder. Wherein, for each cylinder, the internal combustion engine comprises: A first intake conduit (3A) and a second intake conduit (3B), the first intake conduit and the second intake conduit leading to the cylinder and both communicating with the same intake manifold (30) to receive air at the same pressure; a first intake valve (VA) and a second intake valve (VB) associated with the cylinder (1) for controlling the air flow from the first intake duct (3A) and the second intake duct (3B), respectively, into the cylinder during each operating cycle of the cylinder; an actuating device (6) for actuating the first intake valve (VA) and the second intake valve (VB) to control the opening movement and the subsequent closing movement of the first intake valve (VA) and the second intake valve (VB) during each operating cycle of the cylinder; and a gaseous fuel supply system comprising at least one injector (H2) associated with each cylinder and configured and arranged to inject gaseous fuel directly into the respective cylinder during each operating cycle of the cylinder, The method is characterized in that it comprises: by means of the actuating means for actuating the first intake valve (VA) and the second intake valve (VB), during each operating cycle of the cylinder, at least under some engine operating conditions, firstly only the opening movement and the successive closing movement of the first intake valve (VA) are controlled, while the second intake valve (VB) remains closed, and then only the opening movement and the successive closing movement of the second intake valve (VB) are controlled, while the first intake valve (VA) remains closed; and The gas fuel supply system is controlled so that: In each operating cycle of each cylinder, when the first intake valve is open, no gaseous fuel is supplied, so that the air flow introduced into the cylinder with the first intake valve (VA) open causes cooling of the cylinder (1), thereby preventing subsequent pre-ignition or back-ignition phenomena, and In each operating cycle of the cylinder (1), due to the cooling obtained during the opening of the first intake valve (VA), gaseous fuel is directly supplied to the cylinder from the moment after the closing of the first intake valve (VA) without the presence of pre-ignition or backfire.

12. The method according to claim 11, characterized in that The gaseous fuel supply system is configured to complete supply of gaseous fuel before the second intake valve (VB) opens during each operating cycle of the cylinder.

13. The method according to claim 11, characterized in that The gaseous fuel supply system is configured to start supply of gaseous fuel after the second intake valve (VB) is opened during each operating cycle of the cylinder.

14. The method according to claim 11, characterized in that The gaseous fuel supply system is configured to complete the supply of gaseous fuel after the second intake valve (VB) has opened during each operating cycle of the cylinder.

15. The method according to claim 11, characterized in that The method includes variable actuation devices of the first intake valve (VA) and the second intake valve (VB), and wherein, under different engine operating conditions, a first operating mode, a second operating mode or a third operating mode is activated by the variable actuation device, in which, at consecutive times, first only the first intake valve (VA) is opened and then only the second intake valve (VB) is opened, in the second operating mode, the first intake valve (VA) and the second intake valve (VB) are opened at the same or relatively close crank angles, and the first intake valve (VA) and the second intake valve (VB) are closed at the same or relatively close crank angles, and in the third operating mode, only the second intake valve (VB) is opened and closed.

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